This invention relates to data retrieval systems and more particularly to a system which is associated with conventional load management systems.
Load management systems are widely used by electrical utilities to accomplish various desirable end results, such as, for controlling peak loading. For example, such load management systems may remotely control interruptible loads such as air conditioners, heaters, and the like by transmitting command signals through the power transmission system or by radio. In addition, valuable data exists within the power transmission system, such as meter readings, load survey information, load distribution equipment status, and the like. However, two-way systems employing conventional load management techniques are not commonly employed because their cost is excessive in relation to the benefits that can be realized. For example, load management systems may employ high frequency or low frequency command signals. High frequency signals, however, tend to become absorbed by system power factor capacitors and accordingly require inductive traps and repeaters to build up the signal to noise ratio. In addition, because such high frequency signals do not readily pass through transformers, the command and information signals have to be applied beyond the last system transformer thereby requiring a signal generator at each substation. In addition, the input energy required for such systems is relatively high. In low frequency systems, information signals tend to be absorbed in various loads within the system. As a result, a relatively high power input and an expensive powerful transmitter is required at each data transmission point.
For these reasons, two-way systems using conventional load management techniques tend to be costly and technically complex thereby requiring considerable maintenance. Also, such systems tend to have limited flexibility. For example, because high frequency systems require injection of the signal at individual substations, load shifting between substations is thereby limited.
Another method of load management employs radio transmitters. These are not satisfactory for two-way systems, however, because of crowded ratio frequencies and range limitations. Also, load management systems employing telephone lines are not favored by electric utilities because control and switching equipment must be located at telephone companies' central offices, which is under the jurisdiction and control of the telephone utility.